In the process of fabricating integrated circuits (IC's) on wafers, the wafers are subjected to many process steps before a finished IC is produced. The wafers are typically processed with a myriad of specialized tools for forming the various features of the IC, with many of the steps repeated several times. Specialized tools utilized in the IC fabrication process include, but are not limited to, photolithography tools, etchers, ashers, photostabilizers, ion implantation equipment and the like. A significant number of these tools expose the wafer or selected portions of the wafer to a plasma.
Typically, the tools that employ a plasma generate the plasma in close proximity to the wafer surface or produce reactants that interact with the wafer, such as for etching of materials, ashing of photoresist, deposition of materials or the like. Plasma tools are also employed for generating light, such as for example, during photostabilization processes, curing processes, charge erasure processes, and the like. Some plasma-mediated processes employ plasma discharges that are either difficult to ignite, or ignite, but do so irreproducibly with variable delays before ignition is achieved. Once ignited, these discharges are typically sustained with lower required voltages or reduced electric fields. Unfortunately, variability in ignition can lead to variability in processing, inefficiencies, and reduced throughput.
In the semiconductor industry, throughput is often a very important issue. With large volumes and low profit margins in the more competitive areas, incremental improvements in throughput can provide the necessary edge to compete successfully. Variability associated with plasma ignition is often a cause for decreased throughput since process times have to be adjusted to account for the variability.
One causal factor for the difficulty in igniting a gas to form a plasma is due to the relatively high pressures of the working gas. Gases generally have a minimum breakdown voltage operating point that corresponds to relatively low pressures, pressures generally less than about 400 torr and more typically about less than 200 torr. As the gas pressure increases, the required voltage, or electric field, needed to break down the gas increases monotonically. This behavior is problematic since some processes benefit from operation at relatively high pressures, even up to atmospheric-type pressure ranges, at which point very high voltages are required to break down the gas.
Another causal factor for difficulty in plasma ignition is the use of electronegative gases or gas mixtures. Electronegative gases are gases that have a high affinity for electron capture, so that it is very difficult for electrons, once generated, to accelerate and create more free electrons from collision to cause the gas to break down. As a result, establishing a well behaved, steady state plasma can be difficult since the electronegative gas atoms or molecules recapture the electrons. Unfortunately, electronegative gases are frequently the gases of choice for plasma mediated processing of semiconductor wafers for the manufacture of IC's.
The specialized tools that utilize plasmas are driven by energy sources such as microwaves, radiofrequency (RF), other high frequency sources, or the like. Ignition efficiency with these energy sources is generally poor. For example, in the case of a microwave driven plasma, microwave power supplied by a magnetron can be reflected back into the magnetrons. The power supplied by a magnetron is coupled to a microwave cavity for generating the plasma. For most plasma processes, the microwave power can range up to 5,000 watts (W) with gas pressures ranging from 0.5 torr to greater than 5 torr. A common microwave operating frequency is 2.45 gigahertz (GHz). Through the center of the microwave cavity is a plasma tube running lengthwise. The tube is open ended so that it has a gas feed port on top and a gas/plasma exhaust opening at the bottom, leading into a wafer-processing chamber. It is through this tube that various processing gases are passed. Typical gases can include oxygen, nitrogen, hydrogen, helium, and mixtures of these, as well as electronegative gases such as CF4, NF3, and CHF3. Water vapor can also be added. The combined flow rates for the process gases can be as high as 5000 standard cubic centimeters per minute (sccm) or higher. After power is supplied to the magnetrons but prior to ignition, there is no plasma load to absorb the power and power is reflected back into the magnetrons. Reflected power results in a reduced efficiency of the tool and also results in potential damage to the magnetron source. Moreover, once ignited, improper tuning of the microwave driven plasma can further exacerbate the problem of reflected power.
Many plasma tools include tuning hardware to optimize ignition of the gas to form the plasma as well as provide optimization of the breakdown voltage during steady state operation. The ability to ignite the gas mixture depends on the multi-dimensional space defined by all of these variables: gas, pressure, flow rate, electric field provided by the microwave power, and tuning of the cavity. The tuning hardware generally includes an adjustable antenna and an adjustable short. The tuning of the microwave cavity is achieved by moving the antenna position into and out of the microwave cavity, and moving the adjustable short (i.e., a conducting end-plate) up and down to define the length of the cavity. Tuning further adds to the delays associated with operating the plasma tool and as a result, affects throughput.
Once ignited, the reflected power depends on the same variables. That is, without the cavity tuned properly for the given load, some portion of the microwave energy generated from the magnetron is reflected from the load and returned to the microwave source. This reflected power occurs because the presence or absence of the plasma changes the load as seen by the microwave circuit, and changes the tuning of the resonant microwave cavity since the material within the microwave cavity (plasma versus no plasma) affects the resonant wavelength for the cavity. As previously discussed, reflected power results in reduced efficiency of the tool and potential damage to the magnetron source. FIG. 1 graphically illustrates an example of the reflected power versus antenna position and adjustable short position. As shown, relatively high values (>30%) of reflected power can be encountered without proper tuning. Moreover, it has been observed that the optimum starting position of the antenna is usually not the same as the optimum position once the plasma has ignited. Hence, hardware and software has to be added in order to accommodate the two different optimum positions, thereby increasing the costs associated with the operation and manufacture of the plasma tool.
While the repositioning of the antenna offers the advantages of ignition over a larger operating regime, and improved operation during an “on” phase, there still remains a need for a more robust ignition system and process so that repositioning is not necessary or is minimized. Antenna positioning and adjustment of the short requires time, which impacts throughput. Moreover, the use of the microwave cavity tuning system affects reliability, and adds to the total cost to manufacture the plasma tool as well as operating costs.